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1
Preheat the oven to 350 and line 2 large baking sheets with parchment paper.
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2
In a medium bowl, whisk the flour with the cocoa powder, baking soda, baking powder and salt.
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3
In a large bowl, using a handheld electric mixer, beat the butter with the brown sugar at medium speed until fluffy, about 2 minutes.
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4
Beat in the egg and vanilla.
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5
At low speed, beat in the dry ingredients and buttermilk in 3 alternating additions; scrape down the side and bottom of the bowl as necessary.
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6
Using a 3-tablespoon ice cream scoop, scoop 12 level mounds onto each baking sheet, about 1 1/2 inches apart.
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7
Bake the cakes in the lower and upper thirds of the oven for about 16 minutes, until risen and firm; shift the pans from top to bottom and front to back halfway through baking.
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8
Transfer the sheets to racks and let the cakes cool completely.
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9
In a large bowl, beat the butter with the confectioners' sugar and vanilla seeds until fluffy.
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10
Beat in the Marshmallow Fluff.
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11
Scrape the filling into a pastry bag fitted with a large plain tip.
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12
Pipe the filling onto the flat sides of 12 of the cakes and close the whoopie pies, pressing to evenly distribute the filling to the edges.
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13
Roll the edges in orange nonpareils and serve.
